Conventional passive vehicle suspensions are not capable of adjusting to the wide range of dynamic conditions which they meet in practice. These conditions include varying road profiles, vehicle speeds and handling conditions. It means that optimum performance of passive suspensions is necessarily limited to one type of road profile and vehicle speed condition.
On the other hand, fully active suspensions, which exploit the developments of microprocessors, transducers and optimal control theory, are capable of maintaining optimum performance over a wide range of operating conditions. Adjustments to the suspension forces can be made by actuators which receive signals via a microprocessor from a range of transducers attached to the vehicle.
Semi-active vehicle suspensions have been developed as viable alternatives to fully active suspensions, which are expensive in terms of hardware and require significant amounts of power in operation. It has been shown that the optimal performance achievable with fully active systems can be approached with well designed semi-active systems. These systems normally incorporate transducers to monitor suspension positions and velocities together with controllable dampers whose settings are adjusted by a microprocessor.
Whereas fully active systems are able to supply energy to the suspension, semi-active systems are not. When there is a demand for power to be put into the suspension (as determined by the controller), semi-active systems switch to a notional zero damping state. At other times in their operation there is a need to dissipate energy and this is achieved by switching the dampers to two or more settings, or more desirably, adjusting them continuously to meet the needs of the controller. Currently, a number of manufacturers have developed switchable damper systems and developments are known to be underway on continuously variable dampers.
In general, the performance of semi-active systems is compromised by changes in vehicle payload. In effect, as payload on the vehicle is increased, the suspension working space is reduced, leading to a reduction in performance. This problem can be overcome by combining with controllable dampers, a self-leveling system which maintains the design working-space. Most existing self-levelling designs are hydropneumatic systems which require additional hardware such as a hydraulic pump.
It is an object of the present invention to provide a self-levelling damper unit for a semi-active vehicle suspension which is self energizing.